Implantable tissue stimulators are utilized to deliver a train of low voltage level electrical pulses to specific nerves or muscles via an implanted lead. Implantable stimulators have been utilized to stimulate nerves in the spinal cord (paresthesia), to stimulate the bladder and control bladder functions, to stimulate the sphincter in control of that bodily function, and to stimulate the brain (for example, to control epileptic episodes). Other implantable devices provide diagnostic monitoring of a patient's condition as well as the delivery of drugs subcutaneously (such as an insulin pump).
With respect to spinal control stimulation nerves in the spinal cord are electrically stimulated with low voltage level, controlled electric current pulses delivered directly on or at the nerve via an implanted lead or leads in order to achieve paresthesia. Spinal cord stimulation is utilized to treat chronic pain of the trunks and limbs as well as for intractable chest angina. For example, spinal cord stimulation has been utilized to treat peripheral vascular disease (PVD).
With respect to all of these implantable stimulators and devices, it is necessary to provide power to the device implanted below the skin. Since the device is subcutaneously implanted in a patient, the power source must support stimulator or device operation for a reasonable period of time in order to reduce further surgical trauma to the patient. If the device cannot operate under its own power, the device must be surgically removed to replace or recharge the power source therein.
Stimulators can be classified in three general areas: radio frequency or RF coupled and powered stimulators, battery powered stimulators and stimulators which combine RF coupling and battery powered systems. The term battery means an electrochemical (primary or secondary) power system.
The RF coupled and powered stimulator does not carry or contain an independent power source. Therefore, the RF coupled stimulator requires an external RF transmitter and a surgically implanted receiver. The RF link transfers stimulation pulses percutaneously through the skin and adjacent tissue layers of the patient from the external RF transmitter to the surgically implanted RF receiver and stimulator device. The transmitter sends stimulation pulses to be applied ultimately to the implanted electrodes plus programming data defining the polarity of each electrode relative to each other to the implanted stimulation device. The implanted receiver obtains these stimulation pulses and programming data, converts the pulses as necessary, delivers the energy contained in each transmitted stimulation pulse to the implanted electrodes as defined by the programming data. The stimulation pulses are inductively coupled emf waves from the external transmitter to the implanted receiver. The common disadvantage of the RF coupled and powered stimulator is that the patient must always wear the external transmitter and antenna (even during sleep) in order for the implanted receiver to deliver stimulation pulses to the targeted tissue. Stimulation therapy ceases the moment the transmitter antenna is withdrawn just a few inches away from the implanted receiver. Although the RF powered and coupled stimulator has this disadvantage, the service life of such an RF coupled and powered stimulator is not limited to the life of a battery contained within a fully implantable stimulation unit. Accordingly, the long term cost of the RF coupled and powered stimulators is less than the battery powered stimulators because the service life of the former is much longer than that of the latter. RF coupled and powered stimulators have been commercially marketed by Medtronics of Minneapolis, Avery laboratories of New York and Neuromed of Fort Lauderdale, Fla.
The battery powered stimulator utilizes a primary, non-rechargeable battery as a power source to power the implanted stimulator. This battery will operate without requiring an external transmitter to recharge or replenish the battery in the implantable stimulator. The battery provides sole and exclusive power to the implanted stimulator continually while the stimulator generates one or more electric stimulation pulses, in a controlled manner, to the target tissue. Of course, the stimulation pulses are delivered to the targeted tissue via implanted leads. An external programmer may be used to non-invasively adjust the stimulation parameters or control values in the implanted stimulator. Programming may be provided through an RF telemetry link. After programming, the stimulator remembers the parameter values (the values are stored in an electronic memory) as long as the battery voltage remains above a minimum voltage level required by the electronics. Unfortunately, the service life of these battery powered stimulators is limited to the battery life. Accordingly, it is necessary to surgically remove and then replace the battery powered implantable stimulators upon depletion of the electrochemically active materials in the battery. This disadvantage (i.e. surgical replacement) increases its long term cost to the patient relative to the aforementioned RF coupled and powered stimulators. The battery powered implantable stimulators do not require an external transmitter to deliver the stimulation electrical pulses. Accordingly, the battery powered implantable stimulators are easier to use and more comfortable than the RF coupled and powered stimulators. Battery powered stimulators have been marketed by Medtronics of Minneapolis, Neuromed of Ft. Lauderdale and Exonix of Miami.
The third category of implantable stimulators include stimulators which combine the RF coupling and powered delivery systems with the battery powered implantable stimulator technology. These types of stimulators enable the patient to carry the implantable stimulator without the necessity of having an external RF coupled unit proximate the implant at all times. However, the stimulator must be surgically replaced after the battery is depleted if use of the external RF transmitter is not desired. This type of stimulator allows RF coupled stimulation at times when wearing the external transmitter is not objectionable, thereby extending battery life. Also, this type of stimulator may allow for continuing RF coupled stimulation after the internal power source is depleted, although some of these RF coupled and battery powered implantable stimulators do not operate if the battery is completely depleted in the implanted stimulator.
U.S. Pat. No. 4,612,934 to Borkan discloses a non-invasive multi-programmable tissue stimulator. The Borkan implantable stimulator includes an external transmitter which transfers power percutaneously through an RF coupling to an implanted stimulator. The implanted stimulator does include a voltage storage circuit and a battery. The voltage storage circuit stores a minimal amount of voltage and electrical energy. Particularly, the Borkan disclosure provides "[t]he output of the detector circuit 22 is stored as voltage Vm in the voltage storage circuit 36 which comprises diode 80, capacitor 82, optional zener diode 83 and resistor 84. Alternatively, a rechargeable voltage source could be substituted for capacitor 82." Column 14, lines 5-9. Obviously, capacitor 82 is used as a filter device and not as a power source. The long term voltage stored in this circuit Vm is applied to a comparator and, when voltage Vm is less than a predetermined reference voltage, the implantable stimulator "goes to sleep," that is, the implantable stimulator stops delivering stimulation pulses to the targeted tissue. The implantable stimulator is "woken up" or activated upon receipt of RF coupled commands in a certain sequence. Accordingly, the voltage storage circuit in the Borkan disclosure simply acts as a temporary voltage storage unit to detect the presence of the RF transmitter and not a long term power supply for the implanted stimulator. The Borkan stimulator is utilized to stimulate tissue for various neurological and muscular disorders.
U.S. Pat. No. 4,690,144 to Rise discloses a wireless transcutaneous electrical tissue stimulator which deliver stimulation pulses to the surface of the patient's skin. It appears that the Rise transcutaneous stimulator is battery powered stimulator controlled by a wireless remote control. The Rise transcutaneous tissue stimulator is utilized to relieve pain and stimulate muscles as necessary.
U.S. Pat. No. 4,424,812 to Lesnick discloses an implantable, externally programmable, microprocessor-controlled tissue stimulator. The Lesnick disclosure does not describe in detail the electrical energy storage device in the implanted stimulator. However, it is apparent that an internal battery is utilized within the Lesnick implantable stimulator. The external RF coupled device is utilized only to program the implantable stimulator. The patient turns on and off the implanted stimulator by placing and removing a hand held magnet which in turn opens and closes a reed switch in the implantable stimulator.
U.S. Pat. No. 4,406,288 to Horwinski discloses a bladder control device and a method therefor. Basically, an implantable stimulator is utilized to stimulate the pelvic muscles and to control the bladder. The implantable stimulator uses a internal battery as an energy storage device.
U.S. Pat. No. 4,702,254 to Zabara discloses a neurocybernetic prosthesis. The preferred embodiment incorporates a battery and associated circuitry in a fully implantable enclosure. An RF coupled powered device is also discussed.
U.S. Pat. No. 4,556,061 to Barreras discloses a cardiac pacer with a battery consumption monitor circuit. This pacing unit, embodied as an implantable stimulator of the heart, utilizes a battery.